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@(#)ec.4 6.4 (Berkeley) 05/16/86

C 5 ec - 3Com 10 Mb/s Ethernet interface "device ec0 at uba0 csr 161000 vector ecrint eccollide ecxint flags 0" The ec interface provides access to a 10 Mb/s Ethernet network through a 3com controller.

The hardware has 32 kilobytes of dual-ported memory on the UNIBUS. This memory is used for internal buffering by the board, and the interface code reads the buffer contents directly through the UNIBUS. The address of this memory is given in the flags field in the configuration file. The first interface normally has its memory at Unibus address 0.

Each of the host's network addresses is specified at boot time with an SIOCSIFADDR ioctl. The ec interface employs the address resolution protocol described in arp (4P) to dynamically map between Internet and Ethernet addresses on the local network.

The interface normally tries to use a ``trailer'' encapsulation to minimize copying data on input and output. The use of trailers is negotiated with ARP. This negotiation may be disabled, on a per-interface basis, by setting the IFF_NOTRAILERS flag with an SIOCSIFFLAGS ioctl.

The interface software implements an exponential backoff algorithm when notified of a collision on the cable. This algorithm utilizes a 16-bit mask and the VAX-11's interval timer in calculating a series of random backoff values. The algorithm is as follows:

5 1. Initialize the mask to be all 1's.

5 2. If the mask is zero, 16 retries have been made and we give up.

5 3. Shift the mask left one bit and formulate a backoff by masking the interval timer with the smaller of the complement of this mask and a 5-bit mask, resulting in a pseudo-random number between 0 and 31. This produces the number of slot times to delay, where a slot is 51 microseconds.

5 4. Use the value calculated in step 3 to delay before retransmitting the packet. The delay is done in a software busy loop.

"ec%d: send error" . After 16 retransmissions using the exponential backoff algorithm described above, the packet was dropped.

"ec%d: input error (offset=%d)" . The hardware indicated an error in reading a packet off the cable or an illegally sized packet. The buffer offset value is printed for debugging purposes.

"ec%d: can't handle af%d" . The interface was handed a message with addresses formatted in an unsuitable address family; the packet was dropped.

intro(4N), inet(4F), arp(4P) The hardware is not capable of talking to itself. The software implements local sending and broadcast by sending such packets to the loop interface. This is a kludge.

Backoff delays are done in a software busy loop. This can degrade the system if the network experiences frequent collisions.